In a diesel engine with a variable displacement turbocharger having a variable nozzle, a supercharging pressure is actively controllable by controlling a turbine rotation speed through an opening degree of the variable nozzle. Therefore, in the thus configured turbocharged diesel engine, a target supercharging pressure is determined on the basis of an engine rotation speed and a fuel injection amount, and the variable nozzle is operated through feedback control such that an actual supercharging pressure that is calculated from a signal of a supercharging pressure sensor becomes the target supercharging pressure.
Incidentally, in control over the turbocharged diesel engine, there are various constraints on hardware or control, associated with an operation amount of the actuator and a state quantity of the engine. When those constraints are not satisfied, there is a possibility of a breakage of hardware or a decrease in control performance. At least part of those constraints are associated with supercharging pressure control, so the target supercharging pressure that is used in the feedback control is set to a value that satisfies those constraints and the response of the engine at the same time.
However, adaptation work for setting the target supercharging pressure for each engine rotation speed and each fuel injection amount is generally carried out under steady operation of the engine. When transitional operation in which the engine rotation speed is increasing is also included, man-hours required for adaptation work are enormous. It is also difficult to adapt the target supercharging pressure without omission on the assumption of all the transitional operation conditions. Therefore, when the engine is in transitional operation, there is a possibility that part of constraints associated with supercharging pressure control are not satisfied. One of such constraints is a constraint associated with a reliability guarantee on hardware. A difference between the target supercharging pressure and the actual supercharging pressure increases during transitional operation, so the opening degree of the variable nozzle is changed such that the actual supercharging pressure is increased by increasing the turbine rotation speed as soon as possible. At this time, an exhaust pressure rapidly increases with a steep change in the opening degree of the variable nozzle, so there is a possibility of a breakage of hardware if the exhaust pressure excessively increases.
A document that discloses the invention having an object of a reliability guarantee on hardware may be Japanese Patent Application Publication No. 2010-185415 (JP 2010-185415 A). In this publication, there is disclosed the invention of supercharging pressure control that is allowed to avoid an overshoot of a supercharging pressure, which can occur during acceleration, in the configuration that controls the opening degree of a variable nozzle through open-loop control. According to the invention disclosed in this publication, during steady operation, a target intake air amount is calculated on the basis of an engine rotation speed and a fuel injection amount, and the target opening degree of the variable nozzle is determined on the basis of the target intake air amount. On the other hand, during acceleration, a target intake air amount deviation is calculated on the basis of a deviation between a target supercharging pressure and an actual supercharging pressure, and the sum of the target intake air amount deviation and the intake air amount is calculated as a supercharger controlling target intake air amount. A target opening degree of the variable nozzle is determined on the basis of the supercharger controlling intake air amount.
However, constraints that may not be satisfied during transitional operation are not only a constraint associated with a reliability guarantee on hardware. The constraints about supercharging pressure control also include a constraint associated with intake sound, a constraint associated with the controllability of an EGR rate, and the like, and those also may not be satisfied during transitional operation. Speaking of intake sound, there is a possibility of occurrence of a large intake sound that reaches occupant's ears due to a steep change in supercharging pressure. Speaking of the controllability of the EGR rate, there is a possibility of a decrease in the controllability of the EGR rate due to an excessively small differential pressure between the exhaust pressure and the supercharging pressure. In this, way, various constraints associated with supercharging pressure control may not be satisfied during transitional operation; however, the invention disclosed in the above publication just satisfies part of the constraints, and no measures are taken against problems associated with the other constraints.
As is apparent from the above, when feedback control over the supercharging pressure is executed with the use of the variable nozzle, it is necessary to take some measures for satisfying various constraints associated with supercharging pressure control even under transitional operation. In those measures, of course, adaptation work that requires enormous man-hours is made unnecessary or the man-hours are desired to be reduced as much as possible if adaptation work is necessary. Because a supercharging pressure control state that can be taken at one time, more specifically, the opening degree of the variable nozzle, which can be taken at one time, is only one, it is not always possible to satisfy all the constraints at the same time. Thus, when there are a plurality of constraints to be satisfied, it is an important object to determine which constraint is given a priority.